So there is now no problem keeping neons with livebearers or rainbows because they have now been adapted for those tanks
When you pull snippets of sentences out of context you can distort things to have almost any meaning. Some of the species profiles on SF covering "x" species that have been commercially bred for decades mention in the water parameter section or maintenance that while the fish will live in "x" parameters they will have more colour or better health or be more likely to spawn or whatever in softer water. The degree to which a species actually can adapt to differing parameters is a very complex subject, and is not likely to be even remotely understood by many in the hobby. There are certain species which do show a remarkable wide tolerance and maintain good health regardless, but there are other species that certainly do not. It is not really surprising, or should not be to anyone with some knowledge of biology, why some species are so tolerable, but that is another huge topic.
No one to my knowledge has ever satisfactorily provided evidence of soft and very soft water species living
better or even as well in hard water than soft water, and no one has found evidence of hard water species thriving equally or better in soft water. There is evidence that fish from soft waters will live well and thrive in such water, regardless of commercial breeding, and there is plenty of evidence that hard water species will only really live well in such water.
This is a scientific hobby, like it or not. We expect to maintain living creatures in an artificial enclosed tank of water. We must understand and accept that regardless, the laws of the natural world respecting chemistry, biology, microbiology, etc will apply to what goes in and occurs within that confined space. There is plenty of proof that the fish will be more likely of having a good life if we provide what they "expect" from their respective environment. That does not mean they may not be as healthy outside that specific environment--they may or may not, depending upon the species, individual fish, and the conditions that are involved. I am certainly not qualified to be absolute, but I accept the knowledge-based advice of those who are, the ichthyologists, biologists, microbiologists that have spent years learning the science.